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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 54.9 | $ 59.5
Chem-Mod LLC [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 4.0 | 4.0
Chem-Mod International LLC [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 2.0 | 2.0
Clean-Coal Investments [Member] | Controlling Interest [Member] | Fourteen 2009 Era Clean Coal Plants [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 3.8 | 5.1
Clean-Coal Investments [Member] | Controlling Interest [Member] | Nineteen 2011 Era Clean Coal Plants [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 39.8 | 43.0
Clean-Coal Investments [Member] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 0.4 | 0.4
Other Investments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 4.9 | $ 5.0

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